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Southwest Airlines isn’t wasting any time adding new routes from Logan International Airport.

The low-cost Dallas airline, which started flying from Boston to Chicago and Baltimore just over two weeks ago, said yesterday that it would add two daily nonstop flights to St. Louis starting Jan. 10.

“Demand in Boston has been ahead of what we predicted,’’ said Southwest spokesman Paul Flaningan. “As some of our competitors are cutting back, we see it creating opportunities for increased service in St. Louis.’’

St. Louis has been an underserved market since Trans World Airlines, which was based in St. Louis, was acquired by American Airlines in 2001, according to George Hobica of Airfarewatchdog.com. In late 2003, American slashed its St. Louis service in half.

“Having frequent nonstop service at a reasonable rate has not been a reality,’’ Hobica said.

Southwest, which doesn’t fly directly to St. Louis from Providence or Manchester, also said yesterday that it’s adding two daily flights to Lambert-St. Louis International Airport from Minneapolis-St. Paul International Airport, in January.

Because of its proximity to Chicago, which Southwest already serves from Boston, St. Louis wasn’t seen as a top choice for early expansion out of Logan. Some airline analysts had predicted Florida. “Typically what you want to do is spread things out,’’ said Rick Seaney of the travel website Farecompare.com.

But the lack of service to St. Louis created an opening, and Southwest is taking advantage of it. The Midwest city can connect Boston passengers to 24 nonstop destinations, including Las Vegas, Phoenix, and Salt Lake City.

The fact that Southwest added a third destination out of Logan so quickly caught some industry analysts off-guard. Southwest added nonstop flights from Minneapolis-St. Paul to Denver a few weeks after starting service from the twin cities in March, but hasn’t expanded beyond the Chicago and Baltimore routes out of LaGuardia Airport that began in June.

“I was not expecting them to drop in a few more nonstops out of Boston, so this is a surprise,’’ said Seaney.

Southwest is in the process of adjusting its schedule, reducing flight frequencies in 92 markets and temporarily dropping service on three routes in January: Manchester, N.H., to Phoenix and Albuquerque to Portland, Ore., which will both be restored in February, and Kansas City, Mo., to Seattle, which will be reinstated in May. Southwest is also increasing service in 42 markets.

As for what comes next, it’s anybody’s guess. Airlines typically look back to determine how they will go forward, Seaney said, “but there’s no history for $140 a barrel oil followed by a 1930s style recession.’’

“You’d have to be crazy to project what’s going to happen in the next three or four months,’’ he said.

Southwest’s St. Louis fares are on sale now for as low as $59 each way. But if you’re looking to meet someone in St. Louis - and fly nonstop on Southwest for $59 to get there - you’ll have to work fast. The sale ends tomorrow.

AirTran Airways spokesman Christopher White said the airline is currently offering a $69 fare from Boston to St. Louis, connecting through Atlanta or Milwaukee, and will match Southwest’s $59 fare for travel starting Jan. 10. American Airlines has three daily direct flights to St. Louis for as low as $98 but has no immediate plans to match Southwest’s sale fare, said spokesman Ned Raynolds.